1. Field of the Invention
This invention relates to an improvement in the method of biologically purifying wastewater and more particular to an improved method for the activated sludge wastewater treatment process.
2. Description of the Prior Art
The activated-sludge process is a wastewater treatment process by which biologically active microorganisms are continually circulated with incoming biologically degradable waste in the presence of oxygen so that the organic material in the sewage is decomposed to an acceptable degree by the action of the natural life processes of bacteria and other microorganisms. This process is characterized by the presence of countless living organisms as active agents of purification, which, living in the aerobic conditions that are carefully maintained, feed upon the impurities in the wastewater and convert these dissolved impurities into additional cellular masses of microorganisms that are readily removable by sedimentation, the product of which is known as activated sludge.
Wastewater treatment plants employing this process consist essentially of a system having a primary aeration chamber or reactor that may be either a tank or, in the case of a variation known as the oxidation ditch, an open channel, into which sewage flows and where mixing and aeration occur. After a detention period in the aeration chamber, the duration of which depends upon the system design characteristics, the reactor effluent goes to a clarifier tank where in a quiescent environment solids suspended in the liquid settle at the bottom of the clarifier. These solids are then taken in the form of the concentrated sludge that collects at the bottom of the clarifier and are readmitted to the primary aeration tank into which untreated wastewater continually is supplied. Supernatant liquids are drawn from the clarifier and the plant as treated wastewater.
An activated-sludge process can be visualized as a fluid bed process in which the microorganisms are distributed and circulated throughout the wastewater while in the reactor, as distinguished from a second class of treatment process known as the trickling filter process. In the former, contact between the biological organisms and the dissolved organic matter in the wastewater is random and dispersed and preferably occurs throughout all regions of the reactor. The microorganisms are continually held in suspension because the wastewater is constantly being agitated by aeration techniques in the reactor either using waterfall methods, whereby water is tossed into and permitted to fall through the air or by injecting air, as by bubbling through the wastewater. Instead, the trickling filter provides fixed surfaces in the form of a bed of crushed stone or gravel upon which the organisms attach and react with the wastewater-organic material as the sewage trickles downwardly through the bed to drains that carry the effluent away.
In the activated sludge process the circulating mixture is aerated wastewater containing dissolved organic waste material, the microorganisms feeding upon the organics and the activated sludge drawn from a clarifier tank is retained in the reactor for a period of time that varies with the requirements of the particular system and according to the requirements of certain parametric modifications of the process. The majority of extended aeration plants provide long-term, complete-mixing activated sludge systems that maintain the microorganisms in the endogenous phase by establishing a low food-to-microorganism ratio in the mixed reactor liquor. Efficiency of the treatment process is, among other variables, a function of the concentration of active biomass in the system, particularly the concentration in the reactor. Periodic escape of biota from the reactor with the concomitant reduction in overall process efficiency have frequently been noted. To maintain the biologically-active organisms within the reactor, clarifier sediment is continually returned to the reactor rather than allowing the microorganisms to exit the system with the clarifier effluent.
A clarifier tank is essentially a settling chamber into which reactor effluent flows following its reactor detention period. A clarifier is reqired because conventionally clarifier influent contains microorganisms in approximately the same concentrations as they exist in the reactor. But it is advantageous for purposes of maintaining a large concentration of microorganisms in the reactor to recover them, in this case by sedimentation at the bottom of a clarifier, and to return them in a substantially more concentrated liquor to the reactor where they can again interact with the wastewater organics. Loss of the biological organisms from the system represents loss of the vital medium that is fundamental to the activated sludge process.
Typically a clarifier has a cylindrical tank portion positioned above a conically-shaped lower portion. Reactor effluent flows into the tank where in a quiescent environment the heavier masses of microorganisms settle and concentrate in the conical portion in a form known as activated sludge. Suitable piping carries the clarifier precipitate back to the reactor where it is introduced to and mixed with the circulating wastewater usually at a point upstream of the aerator and downstream of the location from which reactor wastewater is taken off for delivery to the clarifier. The treated wastewater remaining in the clarifier above the precipitate is taken off and discharged from the plant.
Conventional activated sludge treatment systems must provide a clarifier of a sufficient size to allow for the volume of clarifier effluent to equal the volume of plant influent over that period of time that is required for an acceptable degree of sedimentation to occur. Moreover, under system load conditions that produce extreme turbidity of the reactor effluent, its clarifier detention period must be increased above the normal duration period to produce a plant effluent that is acceptable in regard to clarity, which condition demands a clarifier of even greater size than normal operation would require. In present practice with systems having a more or less uniform concentration of suspended solid matter in the reactor-based fluid and in the reactor effluent, the piping and pump capacities must necessarily be increased above their capacity requirements if suspended solids were to remain in the reactor and the system were to produce an equivalent volume of treated wastewater. Less power to pump a lesser volume of activated sludge would be required, yet the volume of treated wastewater produced will be maintained with plants employing this invention.